1. Field of the Invention
This invention relates to a security element for a security document such as a bank note, ID card or the like comprising a translucent carrier material having an opaque layer with gaps in the form of characters, patterns or the like recognizable in transmitted light, and a magnetic layer disposed below the opaque layer.
2. Description of the Related Art
It has been known for some time to provide security documents with plastic security threads having a magnetic coating and thus serving as a machine-readable security feature (DE 16 96 245 A1, EP 0 310 707 A1).
To increase the forgery-proofness of this proven security feature further, it has also been proposed to provide the magnetic coating on the carrier material in discontinuous form. For example EP 0 407 550 A1 describes a security document with an embedded security thread provided with a binary code consisting of magnetic material. Certain bit lengths are defined which are constant over the total length of the strip. The coating of a bit length with magnetic material corresponds for example to a 1 while a bit length without magnetic material corresponds to a 0. The binary code known from EP 0 407 550 A1 is characterized in that it is composed of alternatingly disposed separation segments and word segments whereby the word portion consists of a certain number of bit lengths and the sequence of binary values of the separation segments must not occur within this word length in order to permit clear detection of the word segments. This security element has the disadvantage, however, that there is no possibility of fast visual checking as is necessary in many situations of daily life.
It has therefore likewise been proposed to combine machine-testable security features with visual features. EP 0 516 790 A1 discloses a security document with such a security element. The security thread described here consists of a transparent plastic carrier layer with a metallic coating in which gaps are provided in the form of characters or patterns, the so-called negative writing. If the thread is present in the paper pulp, these gaps and the metallic surroundings are hardly visible when viewed in reflected light. When viewed in transmitted light, however, the transparent gaps stand out in strong contrast from their opaque surroundings and are thus easily recognized. At the same time the security element has a magnetic coating provided e.g. below the metal layer in the edge areas of the thread and symmetrically to the gaps along the running direction of the element in the document.
Starting out from this prior art the invention is based on the problem of providing a security element which offers increased protection from forgery while at the same time being easy to produce.
The solution to this problem can be seen in the independent claims. Developments are the object of the subclaims.
The invention starts out from the idea of combining the advantages of visually testable negative writing and machine-readable magnetic coding and ensuring increased protection from forgery relative to the individual security features via their special mutual association on the security element.
The inventive security element therefore consists according to a first embodiment of an at least translucent plastic layer having an opaque layer with gaps and a magnetic coding, the gaps being disposed in the magnetic layer-free areas of the element. The special geometrical arrangement of the gaps and the magnetic areas can consist for example of a nesting of coding and gaps, i.e. the magnetic layer-free portions of the coding are used to dispose the visually testable negative characters in this area. Additionally, it is also possible, however, to provide on the security element a further machine-readable feature such as a further magnetic layer which of course must not impair the testability of the other features.
According to a preferred embodiment the security element has the form of a thread or strip which is embedded at least partly in a document material such as bank note paper, or can also be disposed on the surface.
If the security element is disposed on the surface of the document material, it is not absolutely necessary to prepare all layers necessary for authenticity detection on a separate carrier. If the security element has an optically effective layer such as a diffraction structure or interference layer elements, it is necessary to smooth the surface of the document material in the area of the security element since surface roughness impairs the optical effect and brilliance of the element. This is frequently done by applying a background layer, for example a lacquer layer. The magnetic material can be additionally brought into this layer.
According to a further embodiment of the invention this background layer can be applied in two passes. A magnetic material is added to the first layer applied and this layer then applied discontinuously, e.g. in the form of a bar code. The second layer is disposed all over the first layer and is composed so as to produce optimum adhesion between document material and further marking layers applied, such as a hologram. Here, too, the security element preferably has the form of a strip.
The following examples will therefore also be described with reference to this preferred form. However, it is equally possible within the scope of the invention to give the security element any other outer contour.